Integrated circuits for battery-operated systems, such as mobile telephones, or for contactlessly operated systems, such as chip cards, can have only a particular minimum power consumption, stipulated by standards, in “standby” or “sleep” mode. During the standby or sleep mode, which are also called power-saving mode, no arithmetic operations or other signal changes are executed within the integrated circuit.
One method for saving energy involves all those parts of the integrated circuit being isolated from the supply voltage in power-saving mode whose electrical operation is not required again until the integrated circuit leaves the power-saving mode and resumes its normal operation or regular operating state. Circuit units which can be readily transferred to a power-saving mode contain, by way of example, circuit elements for producing control signals whose storing elements are permitted to lose their information in the power-saving mode because they are automatically set to a particular initial state (reset) before the regular operating state is resumed.
There are also circuit units, such as parts of the CPU (Central Processing Unit) in the integrated circuit, whose isolation from the supply voltage would result in a loss of energy and/or time or even in a loss of information when this circuit unit is put into the regular operating state again. Such circuit units include, by way of example, SRAMs, cache memories, register files and parts of data paths. Said circuit units contain instructions and data for the CPU and also its intermediate results and state registers, such as the “program status word” (PSW) and the “program counter” (PC).